Heat 2 teaspoons oil in a large skillet over medium-high heat and add mushrooms. Cook until mushrooms are tender, remove with a slotted spoon, cover and set aside. Liberally season roast with salt and pepper and add to the skillet. Brown roast on all sides, 8 to 10 minutes and transfer roast to slow cooker.

Reduce heat to medium, add remaining 2 teaspoons oil to skillet, along with onions, celery, carrots and garlic. Cook, stirring occasionally, until lightly browned, about 4 minutes and transfer to the slow cooker.

Increase heat to high, add red wine to the empty skillet, scraping up any browned bits with wooden spoon and simmer for 5 minutes. Add tomatoes and broth, and bring to a boil. Add pepper flakes, bay leaves and thyme. Arrange potatoes around the roast in the slow cooker and transfer the wine mixture to the slow cooker.

Set slow cooker to high, cover, and cook until tender, 6 to 7 hours. Alternatively, cook on low for 9 to 10 hours. Check the temperature with a meat thermometer about 2 hours before the roast is nearing ready to prevent overcooking. It will be ready (well done) at 160 degrees F.

Transfer roast to carving board; loosely tent with foil to keep warm. Discard bay leaves and remove potatoes. Allow liquid in pot to settle, about 5 minutes, then use wide spoon to skim fat off surface. Puree the liquids and solids using an immersion blender or in batches in a blender or food processor. Stir in parsley and season to taste with salt and pepper and return to the slow cooker with the mushrooms and peppers. Let heat for several minutes.

Remove strings from roast and cut into half-inch thick slices. Transfer meat and potatoes to a serving platter. Pour about 1 cup gravy over meat to moisten. Serve more gravy separately.